Multilayer ceramic substrate, method for producing same, and electronic component

ABSTRACT

A multilayer ceramic substrate includes an inner layer portion and surface portions that sandwich the inner layer portion in the stacking direction and have an increased transverse strength because of the surface layer portion having a thermal expansion coefficient less than that of the inner layer portion. At least one of the surface portions covers peripheries of main-surface conductive films arranged on a main surface of an inner portion so as to leave central portions of the main-surface conductive films exposed, so that the main-surface conductive films function as via conductors, thereby eliminating the need to provide a via conductor in the surface portions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilayer ceramic substrate, amethod for producing a multilayer ceramic substrate, and an electroniccomponent including a multilayer ceramic substrate. In particular, thepresent invention relates to a multilayer ceramic substrate having alaminated structure including surface portions and an inner layerportion, in which each of the surface portions has a thermal expansioncoefficient less than that of the inner portion, so that the multilayerceramic substrate has improved strength.

2. Description of the Related Art

A multilayer ceramic substrate is described in, for example, JapaneseUnexamined Patent Application Publication No. 6-29664. JapaneseUnexamined Patent Application Publication No. 6-29664 discloses alow-temperature co-fired multilayer ceramic substrate including glassand a crystalline material, in which surface portions have a thermalexpansion coefficient less than that of an inner layer portion, and inwhich the total thickness of the surface portions arranged on both sidesthereof is less than the thickness of the inner layer portion. JapaneseUnexamined Patent Application Publication No. 6-29664 discloses that theuse of such a structure generates compressive stresses in both surfaceportions during a cooling step after firing, thereby improving thetransverse strength of the multilayer ceramic substrate.

The inventor of the present invention discovered that when aglass-ceramic material including an MO—SiO₂—Al₂O₃—B₂O₃-based glass(wherein MO is at least one selected from CaO, MgO, SrO, and BaO) and analumina powder is used as a material defining surface portions of amultilayer ceramic substrate as described above, the multilayer ceramicsubstrate has an improved transverse strength while having outstandingelectrical properties.

However, the multilayer ceramic substrate includes conductive patternshaving via conductors arranged so as to pass through the surfaceportions. It was discovered that when the via conductors arranged so asto pass through the surface portions are made of a low-resistivityAg-based material and when the surface portions are made of aglass-ceramic material, Ag diffuses into the surface portions to formvoids around the via conductors. It was also discovered that, inparticular, when the glass-ceramic material including anMO—SiO₂—Al₂O₃—B₂O₃-based glass and the alumina powder is used, Agreadily diffuses into the surface portions.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a defect-free multilayer ceramic substratehaving a high transverse strength even when a Ag-based material is used,a method for producing the multilayer ceramic substrate, and anelectronic component including the multilayer ceramic substrate.

A preferred embodiment of the present invention provides a multilayerceramic substrate having a laminated structure including an inner layerportion and surface portions that are in contact with main surfaces ofthe inner layer portion, the inner layer portion being arranged betweenthe surface portions in the stacking direction, in which the surfaceportions have a thermal expansion coefficient less than that of theinner layer portion, the surface portions are made of a glass-ceramicmaterial including glass and a ceramic powder, and the inner layerportion includes a conductive pattern made of a Ag-based material. Toovercome the problems described above, the conductive pattern preferablyincludes main-surface conductive films arranged on one or both of themain surfaces of the inner layer portion, and at least one of thesurface portions covers peripheries of the main-surface conductive filmsso as to leave a central portion of the main-surface conductive filmsexposed.

More specifically, preferred embodiments of the present invention areparticularly advantageously applied when the glass-ceramic materialdefining the surface portions includes an MO—SiO₂—Al₂O₃—B₂O₃-basedglass, where MO represents at least one selected from CaO, MgO, SrO, andBaO, and an alumina powder.

Preferably, the difference in thermal expansion coefficient between theinner layer portion and the surface portions is at least about 1.0ppmK⁻¹, and the content of a component common to both a materialdefining the inner layer portion and a material defining the surfaceportions is at least about 75% by weight. In this case, more preferably,the difference in thermal expansion coefficient between the inner layerportion and the surface portions is about 4.3 ppmK⁻¹ or less.

In the multilayer ceramic substrate according to a preferred embodimentof the present invention, the glass included in the material definingthe surface portions preferably includes about 34% to about 73% byweight of SiO₂, about 14% to about 41% by weight of MO, about 0% toabout 30% by weight of Al₂O₃, and about 0% to about 30% by weight ofB₂O₃, for example.

In a method for the multilayer ceramic substrate according to apreferred embodiment of the present invention, a glass-ceramic pasteincluding glass and a ceramic powder is prepared. A plurality of innerceramic green sheets for forming inner layers are prepared. A conductivepattern made of an Ag-based material is formed on a specific sheet ofthe plurality of inner ceramic green sheets.

Next, a step of forming a laminate including an inner layer portion andsurface portions is performed, the inner layer portion being formed bystacking the plurality of inner ceramic green sheets, and the surfaceportions being formed by applying the glass-ceramic paste to outwardfacing surfaces of the outermost sheets of the inner ceramic greensheets defining the inner layer portion. A step of firing the laminateis then performed.

In the method for producing the multilayer ceramic substrate accordingto preferred embodiments of the present invention, the foregoingconductive pattern preferably includes main-surface conductive filmsformed on one or both of the outward facing surfaces of the outermostsheets of the inner ceramic green sheets defining the inner layerportion, and at least one of the surface portions of the laminate isformed so as to cover peripheries of the main-surface conductive filmsand to expose a central portion of the main-surface conductive films.

The method for producing the multilayer ceramic substrate according topreferred embodiments of the present invention is particularlyadvantageously applied when the glass ceramic paste includes anMO—SiO₂—Al₂O₃—B₂O₃-based glass (wherein MO represents at least oneselected from CaO, MgO, SrO, and BaO) and an alumina powder.

Preferably, the method for producing the multilayer ceramic substrateaccording to preferred embodiments of the present invention furtherincludes the steps of preparing a constraining ceramic green sheet, theconstraining ceramic green sheet including an inorganic material that isnot sintered at a temperature at which the glass-ceramic paste and theinner ceramic green sheets are sintered, and forming a compositelaminate by arranging the constraining ceramic green sheet on at leastone main surface of the laminate. In this case, in the step of firingthe laminate, the laminate is fired at a temperature at which theglass-ceramic paste and the inner ceramic green sheets are sintered butat which the constraining ceramic green sheet is not sintered.

In the multilayer ceramic substrate according to preferred embodimentsof the present invention, although the peripheries of the main-surfaceconductive films arranged on one or both of the main surfaces of theinner layer portion are covered with at least one of the surfaceportions, the central portions thereof are exposed. Thus, themain-surface conductive films provides a function substantially similarto that of the via conductors arranged in the at least one of thesurface portions, thereby eliminating the need to form a via conductorin the surface portions. Even when the conductive pattern including themain-surface conductive films is made of the Ag-based material and thesurface portions are made of the glass-ceramic material including glassand the ceramic powder, the defect-free multilayer ceramic substratehaving a high transverse strength is produced. The reason for this isbelieved to be as follows. A low diffusion of Ag occurs in the surfaceportions due to a small contact area between the main-surface conductivefilms and the surface portions, and at least one of the surface portionsis constrained by the constraining ceramic green sheet when what is anon-shrinkage process is performed with the constraining ceramic greensheet.

According to preferred embodiments of the present invention, even whenthe surface portions are made of the glass-ceramic material, in which Agreadily diffuses, containing a MO—SiO₂—Al₂O₃—B₂O₃-based glass and thealumina powder, the defect-free multilayer ceramic substrate having ahigh transverse strength is produced.

In the multilayer ceramic substrate according to preferred embodimentsof the present invention, since the peripheries of the main-surfaceconductive films are covered with the at least one of the surfaceportions, the bond strength between the exposed main-surface conductivefilms and the multilayer ceramic substrate is increased.

In the multilayer ceramic substrate according to a preferred embodimentof the present invention, a difference in thermal expansion coefficientbetween the inner layer portion and the surface portions of at leastabout 1.0 ppmK⁻¹ effectively prevents warpage of the multilayer ceramicsubstrate. It is believed that the reason for this is that when thedifference in thermal expansion coefficient is increased to at leastabout 1.0 ppmK⁻¹, warpage is corrected because an in-plane stresscausing warpage of the multilayer ceramic substrate is less than thestresses acting on the front and back surfaces in the planar directiondue to the difference in thermal expansion coefficient.

In the multilayer ceramic substrate according to preferred embodimentsof the present invention, when the content of the component common toboth of the material defining the surface portions and the materialdefining the inner layer portion is at least about 75% by weight, asufficient bond strength is obtained between the surface portions andthe inner layer portion. Thus, as described above, even when thedifference in thermal expansion coefficient between the inner layerportion and the surface portions is at least about 1.0 ppmK⁻¹, theoccurrence of defects, such as delamination and voids, for example, isprevented.

In the multilayer ceramic substrate according to a preferred embodimentof the present invention, when the difference in thermal expansioncoefficient between the inner layer portion and the surface portions isabout 4.3 ppmK⁻¹ or less, the occurrence of defects, such asdelamination and voids, for example, due to the difference in thermalexpansion coefficient is more reliably prevented.

In the method for producing the multilayer ceramic substrate accordingto a preferred embodiment of the present invention, in the step offorming the laminate, the glass-ceramic paste is applied so as to coverthe peripheries of the main-surface conductive films and to expose thecentral portions of the main-surface conductive films, thereby formingthe surface portions, and the main-surface conductive films function insubstantially the same manner as the via conductors. Thus, there is noneed for a step of forming a via conductor. For example, there is noneed to perform the steps of preparing a green sheet defining thesurface portion, forming through holes therein, and filling a conductivepaste into the through holes. Therefore, the production process of themultilayer ceramic substrate is simplified.

In the method for producing the multilayer ceramic substrate accordingto a preferred embodiment of the present invention, the use of theconstraining ceramic green sheet prevents shrinkage in the direction ofthe main surface during the firing of the laminate. As a result,undesired deformation of the multilayer ceramic substrate is preventedto thereby improve the dimensional accuracy. Furthermore, delaminationbetween the inner layer portion and the surface portions is much lesslikely to occur during the firing.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a multilayer ceramicsubstrate 1 according to a preferred embodiment of the presentinvention.

FIGS. 2A to 2C illustrate a multilayer ceramic substrate according toanother preferred embodiment of the present invention and a method forproducing the multilayer ceramic substrate, wherein FIGS. 2A and 2B areschematic cross-sectional views of states in the course of theproduction of the multilayer ceramic substrate, and FIG. 2C is a frontelevational view of an electronic component including the multilayerceramic substrate and is a cross-sectional view with respect to themultilayer ceramic substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic cross-sectional view of a multilayer ceramicsubstrate 1 according to a preferred embodiment of the presentinvention.

The multilayer ceramic substrate 1 has a laminated structure includingan inner layer portion 2 and surface portions 3 and 4 which are incontact with main surfaces of the inner layer portion 2 and betweenwhich the inner layer portion 2 is arranged in the stacking direction.The inner layer portion 2 includes a conductive pattern 5 made of anAg-based material. The conductive pattern 5 includes main-surfaceconductive films 6 arranged on one main surface of the inner layerportion 2, an inner conductive film 7 arranged in a central portion ofthe inner layer portion 2 in the thickness direction, and via conductors8 arranged to connect the main-surface conductive films 6 and the innerconductive film 7. The surface portion 3 covers the periphery of each ofthe main-surface conductive films 6 while leaving the center of eachmain-surface conductive film 6 exposed. Thus, the main-surfaceconductive films 6 provides a function that is substantially the same asthat of via conductors arranged in the surface portion 3. Thiseliminates the need to form a via conductor in the surface portion 3.

Among conductive films (not shown in FIG. 1) arranged on the mainsurfaces of the inner layer portion 2, if the conductive films provideonly interconnections in the same plane and do not function as viaconductors, the surface portions 3 and 4 preferably cover substantiallyall of the conductive films. That is, not all of the conductive filmsarranged on the main surfaces of the inner layer portion 2 are the“main-surface conductive films” as defined in preferred embodiments ofthe present invention.

The surface portion 3 preferably covers the periphery of eachmain-surface conductive film 6 as described above. Alternatively, thesurface portion 3 may not entirely cover the periphery of eachmain-surface conductive film 6. That is, the surface portion 3 maypartially cover the periphery of each main-surface conductive film 6.

In the multilayer ceramic substrate 1, the surface portions 3 and 4 havea thermal expansion coefficient less than that of the inner layerportion 2. Preferably, the difference in thermal expansion coefficientbetween the inner layer portion 2 and the surface portions 3 and 4 is atleast about 1.0 ppmK⁻¹, for example. Furthermore, the content of acomponent common to both a material defining the surface portions 3 and4 and a material defining the inner layer portion 2 is preferably atleast about 75% by weight, for example.

This configuration provides a high transverse strength in the multilayerceramic substrate 1, effectively prevents warpage of the multilayerceramic substrate 1, and prevents the occurrence of defects, such asdelamination and voids, for example, at interfaces between the innerlayer portion 2 and the surface portions 3 and 4.

In particular, with respect to warpage, it was discovered that whenusing a method for applying an in-plane compressive stress to each ofthe surface portions 3 and 4 according to preferred embodiments of thepresent invention, a difference in thermal expansion coefficient betweenthe inner layer portion 2 and the surface portions 3 and 4 of at leastabout 1.0 ppmK⁻¹ significantly reduces warpage of the multilayer ceramicsubstrate 1. That is, the relationship between the amount of curvatureand the difference in thermal expansion coefficient was determined asfollows. A difference in thermal expansion coefficient of less thanabout 1.0 ppmK⁻¹ produces a substantially constant amount of curvature,at a difference in thermal expansion coefficient at about 1.0 ppmK⁻¹,the amount of curvature is significantly changed to approximately zero,and a difference in thermal expansion coefficient of more than about 1.0ppmK⁻¹ produces a substantially constant amount of curvature. It isbelieved that the reason for this is that warpage is corrected becausean in-plane stress causing warpage of the multilayer ceramic substrate 1is less than the stresses acting on the front and back surfaces in theplanar direction caused by the difference in thermal expansioncoefficient.

When a component is mounted on or a resin coating is formed on a surfaceof the multilayer ceramic substrate 1, the shrinkage of solder, anadhesive, or the coating resin disadvantageously causes warpage of themultilayer ceramic substrate 1. To overcome the problem, when using themethod for applying an in-plane compressive stress to each of thesurface portions 3 and 4, it was discovered that a difference in thermalexpansion coefficient between the inner layer portion 2 and the surfaceportions 3 and 4 of at least about 1.0 ppmK⁻¹ significantly reduceswarpage of the multilayer ceramic substrate 1. That is, the relationshipbetween the amount of curvature and the difference in thermal expansioncoefficient was determined to be as follows. At a difference in thermalexpansion coefficient of less than about 1.0 ppmK⁻¹, the amount ofcurvature decreases with an increasing difference in thermal expansioncoefficient, and a difference in thermal expansion coefficient of atleast about 1.0 ppmK⁻¹ provides a substantially constant amount ofcurvature. In addition, it is believed that warpage is corrected becausean in-plane stress causing warpage of the multilayer ceramic substrate 1is less than the stresses acting on the surface portions in the planardirection attributed to the difference in thermal expansion coefficient.

The difference in thermal expansion coefficient between the inner layerportion 2 and the surface portions 3 and 4 is preferably about 4.3ppmK⁻¹ or less, for example. This reliably prevents the occurrence ofdefects, such as delamination and voids, for example, at the interfacesbetween the inner layer portion 2 and the surface portions 3 and 4 dueto the difference in thermal expansion coefficient.

The surface portions 3 and 4 are made of a glass-ceramic materialcontaining glass and a ceramic powder. In a preferred embodiment of thepresent invention, the surface portions 3 and 4 are preferably made of aglass-ceramic material containing a MO—SiO₂—Al₂O₃—B₂O₃-based glass, forexample, where MO represents at least one selected from CaO, MgO, SrO,and BaO, and an alumina powder as a filler. In this case, the weightratio of SiO₂ to MO in the glass is preferably in the range of about23:7 to about 17:13, for example. A material defining the inner layerportion 2 preferably includes glass including SiO₂ and MO, for example,in which the ratio of SiO₂ to MO is preferably in the range of about19:11 to about 11:19, for example.

More preferably, glass included in the material defining the surfaceportions 3 and 4 has a SiO₂ content of about 34% to about 73% by weight,for example, and glass contained in the material defining the innerlayer portion 2 has a SiO₂ content of about 22% to about 60% by weight,for example.

Such glass compositions described above are suitable for achieving adifference in thermal expansion coefficient between the inner layerportion 2 and the surface portions 3 and 4 of at least about 1.0 ppmK⁻¹and for achieving a common component content of at least about 75% byweight.

The SiO₂ component in glass contributes to a reduction in the thermalexpansion coefficient. The MO component contributes to an increase inthermal expansion coefficient.

Including an appropriate amount of glass crystallized in the course offiring is preferable to obtain good mechanical strength properties.Thus, the glass composition is preferably close to the composition ofcrystallized glass. For example, in the MO—SiO₂—Al₂O₃—B₂O₃-based glass,MAl₂Si₂O₈ and MSiO₃ are readily crystallized. Thus, the ratio of SiO₂ toMO is preferably adjusted such that the glass has a composition similarto the crystal composition. Accordingly, the ratio of SiO₂ to MO in theglass composition of the surface portions 3 and 4 is preferably close to2, for example, in order to reduce the thermal expansion coefficient.The ratio of SiO₂ to MO in the glass composition of the inner layerportion 2 is preferably close to 1, for example, in order to increasethe thermal expansion coefficient.

The proportion of MO in the glass composition of the inner layer portion2 is preferably greater than that of the surface portions 3 and 4, suchthat the inner layer portion 2 is susceptible to erosion by platingtreatment after firing. However, the inner layer portion 2 is notexposed to the outside and thus is less subjected to fatal damage causedby the erosion.

To increase the difference in thermal expansion coefficient, anexcessively high content of SiO₂ in the glass included in the surfaceportions 3 and 4 causes insufficient sintering due to an inadequatelyreduced glass viscosity. An excessively high content of MO results in aninsufficient difference in thermal expansion coefficient.

Furthermore, to increase the difference in thermal expansioncoefficient, an excessively high content of MO in the glass in the innerlayer portion 2 causes insulation failure due to a reduced resistance tomoisture. An excessively high content of SiO₂ results in an insufficientdifference in thermal expansion coefficient.

Accordingly, the ratio of SiO₂ to MO in the glass included in each ofthe inner layer portion 2 and the surface portions 3 and 4 is preferablyin the ranges disclosed above.

Glass included in the material defining the surface portions 3 and 4more preferably includes about 34% to about 73% by weight of SiO₂, about14% to about 41% by weight of MO, about 0% to about 30% by weight ofB₂O₃, and about 0% to about 30% by weight of Al₂O₃, for example. In thiscase, glass included in the material defining the inner layer portion 2preferably includes about 22% to about 60% by weight of SiO₂, about 22%to about 60% by weight of MO, about 0% to about 20% by weight of B₂O₃,and about 0% to about 30% by weight of Al₂O₃, for example. The reasonfor this is described below.

B₂O₃ functions an appropriate viscosity to glass such that sinteringproceeds smoothly during firing. An excessively high amount of B₂O₃results in excessive baking due to an excessively low viscosity, therebyforming pores on the surface which are likely to cause insulationfailure. A reduction in the amount of B₂O₃ is likely to increase theviscosity, and thus, reduce sinterability. Thus, at least about 1% byweight of B₂O₃, for example, is preferably included.

Al₂O₃ functions as a component defining the crystallized phase in thesurface portions 3 and 4. At an excessively high amount of Al₂O₃, thecrystallization does not easily occur. At an excessively low content ofAl₂O₃, the crystallization does not easily occur. Thus, at least about1% by weight of Al₂O₃, for example, is preferably incorporated.

Al₂O₃ functions to improve the chemical stability of glass, thusimproving the resistance to plating and moisture in the inner layerportion 2 having a relatively large MO content. Al₂O₃ provides anintermediate contribution between SiO₂ and MO to the thermal expansioncoefficient. Thus, an excessively high amount of Al₂O₃ eliminates thedifference in thermal expansion coefficient.

The material defining the surface portions 3 and 4 more preferablyincludes about 30% to about 60% by weight of an alumina powder as afiller, for example. The material defining the inner layer portion 2more preferably includes about 40% to about 70% by weight of the aluminapowder as a filler, for example. The reason for this is described below.

The alumina filler contributes to improved mechanical strength. Anexcessively low content of the alumina filler provides insufficientstrength. In particular, when the inner layer portion 2 that issubjected to a tensile stress does not have sufficient mechanicalstrength, damage occurs at the inner layer portion 2. That is, theeffect of the surface portions 3 and 4 reinforced by the compressivestresses is not sufficiently provided. Thus, the inner layer portion 2has an alumina filler content greater than that of the surface portions3 and 4 and has increased strength, so that the inner layer portion 2can withstand a greater difference in thermal expansion coefficient.Therefore, the effect of the reinforced surface portions 3 and 4 issufficiently provided.

The alumina filler provides an intermediate contribution between glassin the inner layer portion 2 and glass in the surface portions 3 and 4to the thermal expansion coefficient. Thus, an excessively high contentof the alumina filler eliminates the difference in thermal expansioncoefficient.

Examples of the filler that may be included in the inner layer portion 2include other ceramic powders, such as a zirconia powder, in addition tothe alumina powder.

In the multilayer ceramic substrate 1, each of the surface portions 3and 4 preferably has a thickness of about 5 μm to about 150 μm, forexample. The reason for this is described below.

Stresses that occur due to the difference in thermal expansioncoefficient act on the interfaces of the inner layer portion 2 and thesurface portions 3 and 4. More specifically, compressive stresses act onthe surface portions 3 and 4. Each compressive stress decreases withincreasing distance from a corresponding one of the interfaces.Meanwhile, a tensile stress acts on the inner layer portion 2. Thetensile stress decreases with increasing distance from a correspondingone of the interfaces. This is because the stresses are relieved withincreasing distance. At a distance greater than about 150 μm,substantially no compressive stress acts on the surface. Thus, each ofthe surface portions 3 and 4 preferably has a thickness of about 150 μmor less, for example.

When each of the surface portions 3 and 4 has a thickness of less thanabout 5 μm, the inner layer portion 2 having reduced strength due to theoccurrence of the tensile stress is located in the vicinity of asurface, i.e., located less than about 5 μm from the surface. Thus,damage is likely to occur at the inner layer portion 2 in the vicinityof the surface. That is, the effect of the surface portions 3 and 4 thatis reinforced by the compressive stresses is not provided. Thus, each ofthe surface portions 3 and 4 preferably has a thickness of at leastabout 5 μm, for example.

FIG. 1, which shows the multilayer ceramic substrate 1 described above,illustrates the basic structure of a multilayer ceramic substrateaccording to a preferred embodiment of the present invention. FIGS. 2Ato 2C show a multilayer ceramic substrate 11 according to anotherpreferred embodiment of the present invention and illustrate morespecific structure of a multilayer ceramic substrate according topreferred embodiments of the present invention and a method forproducing the multilayer ceramic substrate. FIG. 2C shows an electroniccomponent 12 including the multilayer ceramic substrate 11. FIGS. 2A and2B show states in the course of the production of the multilayer ceramicsubstrate 11 shown in FIG. 2C.

Referring to FIG. 2C, the multilayer ceramic substrate 11 has alaminated structure including an inner layer portion 13, a first surfaceportion 14, and a second surface portion 15, the inner layer portion 13being arranged between the first and second surface portions 14 and 15in the stacking direction. The inner layer portion 13 includes at leastone inner ceramic layer 16.

The multilayer ceramic substrate 11 includes a conductive pattern 19made of an Ag-based material. The conductive pattern 19 includesmain-surface conductive films 20 and 21 arranged on main surfaces of theinner layer portion 13, a plurality of inner conductive films 22arranged so as to define a passive element, e.g., a capacitor or aninductor, or to establish interconnection, such as electricalconnection, for example, between elements in the inner layer portion 13,and a plurality of via conductors 23.

The first surface portion 14 covers the periphery of each of themain-surface conductive films 20 so as to leave the central portion ofeach main-surface conductive film 20 exposed. The second surface portion15 covers the periphery of each of the main-surface conductive films 21so as to leave the central portion of each main-surface conductive film21 exposed.

Chip components 24 and 25 are mounted on one main surface of themultilayer ceramic substrate 11, the chip components 24 and 25 beingelectrically connected to the exposed central portions of themain-surface conductive films 20. Thereby, the electronic component 12including the multilayer ceramic substrate 11 is produced. Themain-surface conductive films 21 exposed at the other main surface ofthe multilayer ceramic substrate 11 define an electrical connection usedto mount the electronic component 12 on a motherboard (not shown).

The foregoing multilayer ceramic substrate 11 is preferably produced asfollows.

Referring to FIG. 2A, a glass-ceramic paste including glass and aceramic powder for forming the surface portions 14 and 15 is prepared.In a more specific preferred embodiment, a glass-ceramic pastepreferably including a MO—SiO₂—Al₂O₃—B₂O₃-based glass and an aluminapowder, for example, is used. For example, glass(CaO:SiO₂:Al₂O₃:B₂O₃=26:60:5:9) and an alumina powder are preferablymixed in a weight ratio of about 60:40, for example. Then about 10 partsby weight of an acrylic binder and about 20 parts by weight of a solvent(terpineol) are preferably added thereto with respect to 100 parts byweight of the total amount of the glass and the alumina powder. Theresulting mixture is preferably wet-mixed under predetermined conditionsto produce a glass-ceramic paste.

A plurality of inner ceramic green sheets 26, which will define theinner ceramic layer 16, are prepared. For example, glass(CaO:SiO₂:Al₂O₃:B₂O₃=45:45:5:5) and an alumina powder are preferablymixed in a weight ratio of about 48:52. Then about 10 parts by weight ofa butyral binder, about 3 parts by weight of a plasticizer, and about 10parts by weight of a solvent (terpineol) are added thereto with respectto 100 parts by weight of the total amount of the glass and the aluminapowder. The resulting mixture is wet-mixed under predeterminedconditions to form a slurry. The inner ceramic green sheets 26 eachpreferably having a thickness of about 50 μm, for example, can beproduced from the slurry.

In this preferred embodiment, constraining ceramic green sheets 27 areprepared, the ceramic green sheets 27 including an inorganic materialthat is not sintered at a temperature at which the glass-ceramic pasteand the inner ceramic green sheets 26 described above are sintered. Forexample, an alumina powder is mixed with an organic solvent. Then about10 parts by weight of a butyral binder and about 3 parts by weight of aplasticizer are added thereto with respect to 100 parts by weight of thealumina powder. The resulting mixture is wet-mixed under predeterminedconditions to form a slurry. The constraining ceramic green sheets 27each preferably having a thickness of about 100 μm, for example, can beproduced from the slurry.

A conductive paste for forming the conductive pattern 19 including themain-surface conductive films 20 and 21, the inner conductive films 22,and the via conductors 23 is prepared. The conductive paste preferablyincludes, for example, about 70% by weight of an Ag powder, about 10% byweight of an acrylic resin, and about 20% by weight of a terpineolsolvent. Alternatively, in place of the Ag powder, an Ag/Pd mixed powdermay be used.

The conductive pattern 19 is formed with the conductive paste on theresulting ceramic green sheets 26 defining the inner layers. Morespecifically, the main-surface conductive films 20 and 21, the innerconductive films 22, and the via conductors 23 are formed on specificinner ceramic green sheets 26, as necessary.

The glass-ceramic paste is applied to outward facing surfaces of theoutermost sheets of the inner ceramic green sheets 26, thereby formingthe surface portions 14 and 15. The surface portions 14 and 15 areformed so as to cover the periphery of each of the main-surfaceconductive films 20 and 21 and leave the central portion of each of themain-surface conductive films 20 and 21 exposed. Each of the surfaceportions 14 and 15 preferably has a thickness of, for example, about 10μm.

These elements described above are stacked in the order and thedirection shown in FIG. 2A. The resulting stack is pressed with a pressmachine to produce a composite laminate 28 shown in FIG. 2B. At thispoint, exposed portions of the main-surface conductive films 20 and 21and outward facing surfaces of the surface portions 14 and 15 areplanarized by pressing, so that the exposed portions of the main-surfaceconductive films 20 and 21 are substantially flush with the outwardfacing surfaces of the surface portions 14 and 15, respectively.Although four sheets are shown for the inner ceramic green sheets 26, inpractice, for example, about 20 sheets are stacked. The compositelaminate 28 includes a laminate 29 to be formed into the multilayerceramic substrate 11, the laminate 29 being arranged between theconstraining ceramic green sheets 27.

Next, the composite laminate 28 is preferably fired at about 900° C., ina firing step. For example, a temperature of about 870° C. is maintainedfor about 10 minutes in the firing step. During the firing step, theconstraining ceramic green sheets 27 constrain the shrinkage of thelaminate 29 in the planar direction. When the firing step is completed,the laminate 29 in the composite laminate 28 is sintered to form themultilayer ceramic substrate 11, and the constraining ceramic greensheets 27 are formed into unsintered porous portions. Removal of theunsintered portions with, for example, an ultrasonic cleaner results inthe multilayer ceramic substrate 11 shown in FIG. 2C.

Subsequently, the chip components 24 and 25 are mounted on themultilayer ceramic substrate 11 as shown in FIG. 2C, thereby completingthe electronic component 12.

The foregoing production method may be modified as follows.

In a first modification of the preferred embodiment of the presentinvention described above, the main-surface conductive films 20 and 21,the inner conductive films 22, and the via conductors 23 are formed onthe inner ceramic green sheets 26, as necessary. These inner ceramicgreen sheets 26 are stacked to form the laminate 29. Then aglass-ceramic paste is applied to the outer surface of the laminate 29,thus forming the surface portions 14 and 15.

In a second modification of the preferred embodiment of the presentinvention described above, unlike the foregoing first modification, inorder to form the surface portions 14 and 15, a glass-ceramic paste isapplied on support films to form patterns to be formed into the surfaceportions 14 and 15. Then the patterns on the support films aretransferred to the outer surface of the laminate 29.

In a third modification of the preferred embodiment of the presentinvention described above, the surface portions 14 and 15 are formed bytransferring as in the second modification. In addition, themain-surface conductive films 20 and 21 are also formed by transferring.

In a fourth modification of the preferred embodiment of the presentinvention described above, instead of using the foregoing constrainingceramic green sheets 27, the laminate 29 is fired without theconstraining ceramic green sheet.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A multilayer ceramic substrate comprising: a laminated structureincluding an inner layer portion and surface portions in contact withmain surfaces of the inner layer portion, the inner layer portion beingarranged between the surface portions in a stacking direction; whereinthe surface portions have a thermal expansion coefficient less than thatof the inner layer portion; the surface portions are made of aglass-ceramic material including glass and a ceramic powder; the innerlayer portion includes a conductive pattern made of an Ag-basedmaterial; the conductive pattern includes main-surface conductive filmsarranged on at least one of the main surfaces of the inner layerportion; at least one of the surface portions covers peripheries of themain-surface conductive films from above so as to leave central portionsof the main-surface conductive films exposed and such that theperipheries of the main-surface conductive films are disposed betweenthe at least one of the surface portions and the inner layer portion. 2.The multilayer ceramic substrate according to claim 1, wherein theglass-ceramic material includes an MO—SiO₂—Al₂O₃—B₂O₃-based glass, whereMO represents at least one selected from CaO, MgO, SrO, and BaO, and analumina powder.
 3. The multilayer ceramic substrate according to claim1, wherein a difference in the thermal expansion coefficients betweenthe inner layer portion and the surface portions is at least about 1.0ppmK⁻¹; and a content of a component common to both a material definingthe inner layer portion and a material defining the surface portions isat least about 75% by weight.
 4. The multilayer ceramic substrateaccording to claim 3, wherein the difference in thermal expansioncoefficient between the inner layer portion and the surface portions isabout 4.3 ppmK⁻¹ or less.
 5. The multilayer ceramic substrate accordingto claim 2, wherein the MO—SiO₂—Al₂O₃—B₂O₃-based glass includes about34% to about 73% by weight of SiO₂, about 14% to about 41% by weight ofMO, about 0% to about 30% by weight of Al₂O₃, and about 0% to about 30%by weight of B₂O₃.
 6. An electronic component comprising the multilayerceramic substrate according to claim 1.